The acoustic material employed in the diaphragm of a loudspeaker is required to have low density, high modulus of elasticity and hence a high rate of propagation of longitudinal waves and large internal losses, for enhancing the reproduction frequency range. With this in view, evolution towards industrial application of a so-called composite diaphragm is now underway using a variety of fibers such as carbon-, aramide-, glass- or polyolefin resin fibers as the reinforcing materials.
Above all, drawn high elastic modulus polyethylene, prepared by a crystal surface growth method, gel spinning-ultradrawing method or a melt draw orientation method is thought to be suitable as the acoustic material, in that it has a lower density and a higher rate of propagation of longitudinal waves. For example, it is shown in the Japanese Patent Publication KOKAI No - 182994/1983 to use polyethylene fibers having the rate of propagation of the longitudinal waves not lower than 4000 m/sec as the acoustic material
It is noted that the aforementioned high elastic modulus polyethylene fibers compare favorably with aluminum in elastic modulus (Young's modulus), but are inferior to polyester in internal losses (tan .delta.), as shown in Table 1 indicating the physical properties thereof, such that it cannot be used directly as the acoustic material, above all, as the loudspeaker diaphragm.
TABLE 1 ______________________________________ Young's tan.delta. modulus method of preparation ______________________________________ polyethylene a 0.013 47 fibrilated crystal fibers b 0.011 82 growth, gel spinning- c 0.014 78 ultra drawing, or melt spinning orienta- tion aluminum 0.008 73 -- polyester 0.053 5 biaxially drawn film ______________________________________
The present invention has been made in view of the above described deficiencies of the prior art and is aimed to provide an acoustic material which is improved in internal losses without impairing the high modulus of elasticity proper to the drawn high elastic modulus polyethylene and which is relatively free from higher harmonic distortion or from fluctuations in the frequency response, that is, crests and valleys, caused by split vibrations, when the acoustic material is used as the diaphragm material.